U.S. Patent Application entitled "AFC System For A Synthesizer Tuning System," Ser. No. 956,955, filed Nov. 2, 1978, by the same inventors and assigned to the same assignee as this application and hereby incorporated by reference, discloses a voltage-controllable reference oscillator for a synthesized television receiver local oscillator (LO). The reference oscillator has a nominal 4 MHz operating frequency and its output is effectively frequency divided and compared, according to well known Phase-Locked Loop (PLL) frequency synthesis techniques, to an appropriately divided sample of the LO signal so as to generate the desired LO frequencies, 101 to 129 MHz on Low Band VHF channels 2 through 6, 221 to 257 MHz on High Band VHF channel 7 through 13, and 517 to 931 MHz on UHF channels 14 through 83. The resulting LO frequency is at all times a rational multiple of the 4 MHz reference oscillator frequency.
The reference oscillator is capable of operation in an injection-locked mode whereby its output frequency is determined by a highly stable 4 MHz crystal oscillator to which the reference oscillator is injection-locked. However, when receiving nonstandard frequency signals such as those provided by some CATV and MATV sources, and home video games it is desirable that the reference oscillator frequency be continuously variable, over a limited range, according to the voltage applied to its tuning voltage control terminal. In the voltage-controlled mode of operation the tuning voltage may be derived from either an AFC control circuit or a manually variable Fine Tuning circuit. The U.S. Pat. Application referred to above illustrates a Fine Tuning circuit, element 5 therein, as a variable resistor coupled between two voltage sources, V1 and V2. Although that circuit is unquestionably effective for its intended purpose, there are instances, as described below, when it is useful to effect the fine tuning function in a somewhat more sophisticated manner. For example, it has been found desirable to maintain the fine tuning range at approximately .+-.1.5 MHz, regardless of the channel on which the receiver is operating. However, because the 4 MHz reference oscillator is multiplied by a variable factor that depends on the desired local oscillator frequency for the selected channel, it is not sufficient to maintain a fixed reference oscillator fine tuning range for all channels. To illustrate, on Channel 2 the nominal local oscillator frequency is 101 MHz and the reference oscillator will be effectively multiplied by a factor of 25.25 to achieve the desired LO frequency. In order to implement a fine tuning range of .+-.1.5 MHz, it is necessary to vary the reference oscillator frequency only approximately .+-.60 KHz (.+-.1.5 MHz/25.25). However, on UHF channel 83 the LO frequency is 931 MHz and the multiplying factor is 232.75. A 60 KHz deviation of the reference oscillator would result in almost a 14 MHz deviation, that is, fine tuning range, of the LO frequency. Such a range, greater than four channel widths, is clearly excessive. For the reference oscillator illustrated referred to above, empirical investigation indicated that limiting the range of fine tuning voltage to 2 to 28 volts on Low Band VHF channels 2 through 6, 6 to 18 volts on High Band VHF channels 7 through 13, and 8 to 12 volts on UHF channels 14 through 83, would maintain adequately uniform fine tuning range on all channels.